After fusion and entry, the Human Immunodeficiency Virus (HIV-1) must navigate its genome through the cytosol and into the nucleus of its target cells, where integration into the host's chromosomal DNA subsequently occurs. Following the completion of reverse transcription, the genome of HIV-1 is contained in a large nucleoprotein complex operationally defined as the viral preintegration complex, or PIC. This assembly of viral RNA and/or DNA, and viral and cellular proteins is capable of traversing the cytoplasm, entering the nucleus, and targeting host chromatin. There, the viral integrase protein (IN), assisted by the action of host enzymes, catalyzes the insertion of viral DNA into that of the host and subsequent repair of the chromosomal interruptions created by the integration event itself
The PIC, and the reverse transcription complexes that precede it in the viral life cycle likely constitute a series of structural and functional intermediates with changing composition. In fact, a variety of nucleoprotein complexes having discrete sedimentation velocities have been isolated from the cytoplasm and nucleus of HIV infected cells, at specific times post-infection. Not surprisingly, attempts to characterize in biochemical fashion the viral and cellular protein components of PICs have yielded disparate results. Integrase is consistently found as a constituent of the PIC, as would be expected for a viral protein that performs obligate catalyses on the viral DNA substrate in both the cytoplasm and nucleus. Other PIC-associated viral proteins include matrix (MA), Vpr, and, less frequently reported, nucleocapsid (NC), capsid (CA), and reverse transcriptase (RT).
A facilitating role in the nuclear import of the viral genome has been attributed to a number of PIC components, including MA, Vpr, IN, and the viral DNA itself Nevertheless, assignment and definition of the relative importance of each remains controversial. While some studies have implicated MA as a mediator of infection in non-dividing, others have shown that such an infection proceeds efficiently in the absence of either karyophilic determinants within MA or the majority of the MA protein. The ability of Vpr to interact with the nuclear pore complex has been well documented, and varying degrees of block to viral replication have been observed in its absence. Yet viruses lacking both Vpr and the karyophilic determinants of MA can still efficiently infect non-dividing cells. More recently, a facilitating role in PIC nuclear import has been attributed the central DNA flap—a discontinuity of the plus-strand of the HIV genome consisting of a short region of overlapping DNA and acting apparently in cis. Viruses lacking the central flap efficiently complete reverse transcription, but exhibit decreased nuclear import of viral DNA. Conversely, reconstitution of central DNA flap synthesis in lentiviral vectors that normally lack the structure increases their ability to transduce a variety of cell and tissue types in vitro and in vivo. However, recent experiments suggest that flap-deficient viruses can exhibit near wild-type levels of nuclear uptake under a variety of experimental conditions.
The final PIC component with reported karyophilic activity to date is integrase. Although an initial study characterized the intracellular localization of an IN-β-galactosidase fusion protein as cytoplasmic, further work using both transient transfection and in vitro nuclear import assays has shown that IN localizes predominantly to the nucleus. One study has reported that the mutations K186Q and Q214L/Q216L prevent the import of isolated IN or of other PIC-associated proteins (and presumably of the PIC itself) in assays performed in vitro, however, others have observed wild-type accumulation of nuclear forms of viral DNA in the presence of these mutations. In a more recent study, a peptide spanning amino acids 161-173 of IN was reported to act as a transferable NLS. Mutations at residues V165 and R166 abrogated the nuclear localization of an IN-pyruvate kinase fusion protein and, in the context of infection, impaired the accumulation of viral DNA in the nuclei of both dividing and non-dividing cells. Again, the interpretation of these results has been challenged and/or reassessed. Thus the precise mechanism of nuclear entry, as well as the identity of the cellular factors and viral determinants that are essential for it, remains elusive.